The primary aim of the proposed research is to further the understanding of how retroviruses (RNA tumor viruses) are assembled and bud from the plasma membrane of the infected cell. Most of the work will be based on avian sarcoma and leukemia viruses as a model system, but part of it also will focus on a murine sarcoma virus that is defective in proteolytic processing of the internal structural protein. Experiments with cross-linking agents will be carried out to continue the investigation of how the gag (internal structural) proteins interact with the membrane of the virus and with other proteins. The small cell-encoded polypeptide we have found recently in avian viruses will be characterized and its role in viral structure and assembly examined. The precursor of the five avian gag proteins will be purified as a native polypeptide from quail cells, and also from E. coli carrying a plasmid engineered to express the gag gene. The interactions of the avian precursor with viral RNA and with lipid will be examined and its possible proteolytic activity tested. The structure of the immature cores of murine sarcoma virus will be studied, and experiments to reconstitute cores from the gag precursor polypeptide, and virus-like particles from cores and lipids will be attempted. In addition, the transforming gag fusion protein of Fujinami sarcoma virus will be purified and its enzymological properties studied.